There has been known a vehicle-mounted communication system mounted in an automobile and provided with at least one piece of vehicle-mounted equipment (node) connected to a bus, and a gateway that relays communication between nodes connected to mutually different buses (see Patent Literature 1).
In this kind of vehicle-mounted communication system, data is transmitted and received by use of a protocol defined for each bus, and as the protocol, a CAN (Controller Area Network) protocol, a LIN (Local Interconnect Network) protocol, or the like, is used.
A general gateway mounted in the automobile outputs data received from the node to a defined bus in accordance with routing information. In the gateway, when data frames are to be outputted to a CAN bus, the data frames are outputted to the CAN bus sequentially in order of reception in the gateway itself from each node (namely, the gateway functions as a FIFO buffer). The CAN bus mentioned here is a bus that transmits and receives data by use of the CAN protocol.
In the CAN protocol, a head portion of a data frame includes a CAN-ID. This CAN-ID is an identifier that identifies a transmission source node and contents of data set in a subsequent region, and is set for each data frame.
In the CAN protocol, when the bus is empty, the nodes transmit data frames to the bus, respectively. However, there are cases where data frames are simultaneously transmitted from the respective nodes to the bus.
In this case, in the CAN protocol, it is defined so as to perform arbitration control in which a data frame with the smallest logical value of the CAN-ID is regarded as a data frame with the highest priority, and the node having transmitted that data frame obtains a transmission right.
Hence in the gateway, in a case where a data frame to be transmitted to the CAN bus (hereinafter referred to as a “transmission target frame”) is received from a bus different from the CAN bus, when the transmission target frame has a low priority, it may continue to lose the arbitration over the transmission right against the other nodes.
When the node continues to lose the arbitration over the transmission right, there occurs a problem where a long period of time is required in the gateway until transmission of the transmission target frame to the CAN bus.
That is, the conventional vehicle-mounted control device has a problem where a retention time until transmission of a data frame to the bus can be long.